How the calculation works
Each device row adds its power at the switch port (the PSE, power sourcing equipment):
- Class maximum (the default): the most a port can supply for that class under IEEE 802.3.
- Watts at the device (optional): the data sheet maximum for the device (the PD, powered device) plus the class’s worst-case cable loss, the difference between the PSE and PD figures in the table below.
| Class | At the switch (PSE) | At the device (PD) | Switch type needed |
|---|---|---|---|
| 0 | 15.4 W | 12.95 W | PoE (802.3af) |
| 1 | 4 W | 3.84 W | PoE (802.3af) |
| 2 | 7 W | 6.49 W | PoE (802.3af) |
| 3 | 15.4 W | 12.95 W | PoE (802.3af) |
| 4 | 30 W | 25.5 W | PoE+ (802.3at) |
| 5 | 45 W | 40 W | PoE++ (802.3bt Type 3) |
| 6 | 60 W | 51 W | PoE++ (802.3bt Type 3) |
| 7 | 75 W | 62 W | PoE++ (802.3bt Type 4) |
| 8 | 90 W | 71.3 W | PoE++ (802.3bt Type 4) |
Class power from IEEE 802.3 as tabulated in Power over Ethernet (Wikipedia, read October 2026). The figures at the device assume the standard’s worst case of 100 meters of cable; shorter runs lose less.
Then:
- Usable budget = switch PoE budget × (1 − reserve %)
- Total draw = the sum of quantity × watts at the switch, for every device the switch type can power
- Headroom = usable budget − total draw
- Ports used = every device, including any powered by an injector
- More that fit, per row = the smaller of headroom ÷ that device’s watts (rounded down) and the free ports
A device whose class is above the switch type is left out of the switch’s total and flagged for a midspan injector or a switch of a higher type. The device type only suggests a starting class (class 2 for phones, 4 for access points, 3 for cameras, access control and other devices): these are planning assumptions, so set each row’s class from the data sheet.
Worked example
A 48-port PoE+ switch with a 740 W PoE budget, powering 30 IP phones at class 2, 6 access points at class 4, 8 cameras at class 3 and 2 door controllers at class 3:
| Devices | Each at the switch | Total |
|---|---|---|
| 30 IP phones, class 2 | 7 W | 210 W |
| 6 access points, class 4 | 30 W | 180 W |
| 8 cameras, class 3 | 15.4 W | 123.2 W |
| 2 access control devices, class 3 | 15.4 W | 30.8 W |
| Total | 544 W |
The switch uses 544 W of 740 W (73.5%), leaving 196 W of headroom. The devices take 46 of 48 ports, so only 2 more access points fit, even though the power would cover 6 (196 ÷ 30 = 6.5): this switch runs out of ports before power.
Swap the access points for a model that needs class 6 and each needs 60 W per port, more than PoE+ supplies. They would need injectors or an 802.3bt switch.
Before you buy
- Check the switch’s PoE budget for the power supply you actually have. Many switches list a higher budget only with a second or larger supply.
- Plan the switch’s own uplinks: they aren’t PoE ports and aren’t counted here.
- PoE devices go down with the switch. Put PoE switches for phones, cameras and door controllers on a UPS; the UPS runtime calculator sizes one from the switch’s input power, which is the PoE draw plus the switch itself, divided by the power supply’s efficiency.
